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21 June, 04:54

A solution is prepared by mixing 50.0 ml of 0.50 m cu (no3) 2 with 50.0 ml of 0.50 m co (no3) 2. sodium hydroxide is slowly added to the mixture. which precipitates first? [ksp (cu (oh) 2) = 2.2 * 10-20, ksp (co (oh) 2) = 1.3 * 10-15]

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  1. 21 June, 05:08
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    When we dilute both molarities of Cu (NO3) and CO (NO3) 2 to half

    to become 0.25 M Cu (NO3) & 0.25 M CO (NO3) 2

    and when the value of Ksp CO (OH) 2 = 1.6 x 10^-15

    and the value of Ksp Cu (OH) 2 = 2.2 x 10^-20

    1) so, for CO (OH) 2 Ksp expression = [CO+2][OH-]^2

    by substitution:

    1.6 x 10^-15 = 0.25 m * [OH-]^2

    ∴ [OH] = 8 x 10^-8

    ∴POH = - ㏒[OH-]

    = - ㏒ 8 x 10^-8

    = 7.1

    ∴ PH = 14 - POH

    PH = 14 - 7.1

    = 6.9

    2) and for Cu (OH) ^2

    Ksp expression = [Cu+2][OH-]^2

    by substitution:

    2.2 x 10^-20 = 0.25 * [OH-]^2

    ∴[OH-] = 2.97 x 10^-10

    POH = - ㏒[OH]

    = - ㏒ (2.97 x 10^-10)

    = 9.5

    ∴PH = 14 - POH

    = 4.5

    ∴ Cu (OH) 2 will precipitate first because it needs less [OH-] and PH values than CO (OH) 2 to reach the Ksp value and reach equilibrium.

    So you have to add more [OH-] than 2.97 x 10^-10 to precipitate Cu (OH) 2

    and you have to add more [OH-] than 8 x 10^-8 to precipitate CO (OH) 2
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